Adaptive pivot for variable displacement vane pump

ABSTRACT

Variable displacement pumps and methods of pumping a fluid are provided. An example pump may include a housing having an inlet and an outlet, a rotor fixed for rotation with a shaft, the shaft rotatably mounted within the housing, and a plurality of radially extending vanes slidably disposed in the rotor. The pump may further include a pivotable ring member defining at least in part a control chamber about the rotor, wherein the ring member is pivotable within the housing to vary an eccentricity of the ring member with respect to the rotor. The ring member is configured to shift a pivot position of the ring member from a first pivot position to a second pivot position displaced from the first pivot position.

INTRODUCTION

Variable displacement oil pumps may employ a pivotable ring member or slide within a housing, which facilitates a change in pump displacement by way of varying an eccentricity of the slide with respect to a pump rotor. The pivotable slide may be biased in a given direction about the axis of the rotor with a spring. The slide may be secured within the housing at a fixed pivot point, and pivoted to a desired position corresponding to a desired displacement of the pump.

The ring member position is typically controlled in part by the biasing of the spring, and application of an external pressure against the spring. Internal pressure may also build within the ring member as rotor speed increases. As rotor speed becomes significant, internal torque placed upon the ring member and other components builds may become similarly significant, far outweighing the available external pressure that can be applied to the ring member. Accordingly, known variable displacement pumps may lose a degree of control of displacement as the internal torque becomes large. This is especially disadvantageous when eccentricity of the ring member (and, thus, pump displacement) is relatively large or close to a maximum of the pump.

Accordingly, there is a need for an improved pump that addresses the above shortcomings.

SUMMARY

In at least some example approaches, a variable displacement pump is provided that includes a housing having an inlet and an outlet, a rotor fixed for rotation with a shaft, the shaft rotatably mounted within the housing, and a plurality of radially extending vanes slidably disposed in the rotor. The pump may further include a pivotable ring member defining at least in part a control chamber about the rotor, wherein the ring member is pivotable within the housing to vary an eccentricity of the ring member with respect to the rotor. The ring member is configured to shift a pivot position of the ring member from a first pivot position to a second pivot position displaced from the first pivot position.

In some examples, a resilient element may be engaged with the ring member, thereby biasing the ring member in a first pivot direction opposed to a second pivot direction, the control chamber positioned such that an external pressure introduced into the control chamber applies a force to the ring member tending to pivot the ring member in the second pivot direction.

In some example approaches, the radially extending vanes define one or more vane chambers configured to develop pressure during operation of the pump, the pressure in the one or more vane chambers imparting a force to the ring member offset from a pivot pin of the ring member by an offset distance, wherein the pivot position is shiftable to reduce the offset distance.

In some examples, the pump includes a pivot pin configured to define the first and second pivot positions. Some of these examples may include a pivot anchor fixed to the ring member.

The housing may, in some examples, define at least first and second undulations corresponding to the first and second pivot positions of the ring member, respectively.

In other examples, the pump may include a cam rotatably fixed within the housing. In some of these examples, the cam defines the first pivot position of the ring member when placed in a first rotational position, and the cam defines the second pivot position of the ring member when rotated from the first rotational position to a second rotational position.

Another example variable displacement pump, includes a housing having an inlet and an outlet, a rotor fixed for rotation with a shaft, the shaft rotatably mounted within the housing, and a plurality of radially extending vanes slidably disposed in the rotor. THE pump may include a pivotable ring member defining at least in part a control chamber about the rotor, wherein the ring member is pivotable within the housing to vary an eccentricity of the ring member with respect to the rotor. The ring member is configured to shift a pivot position of the ring member from a first pivot position to a second pivot position displaced from the first pivot position. The pump may further include a resilient element engaged with the ring member, thereby biasing the ring member in a first pivot direction about the first and second pivot positions.

In some of these examples, the radially extending vanes define one or more vane chambers configured to develop pressure during operation of the pump, the pressure in the one or more vane chambers imparting a force to the ring member offset from a pivot pin of the ring member by an offset distance, wherein the pivot position is shiftable to reduce the offset distance.

Some examples pumps may further include a pivot pin configured to define the first and second pivot positions.

In some example approaches, the housing defines first and second undulations corresponding to the first and second pivot positions of the ring member, respectively.

In other examples, the pump may include a cam rotatably fixed within the housing.

In at least some examples, a method of pumping a fluid includes providing a housing having an inlet and an outlet, rotatably mounting a rotor within the housing upon a shaft, positioning a plurality of radially extending vanes in the rotor, the vanes each slidably disposed in the rotor, and defining a control chamber about the rotor with a pivotable ring member, wherein the ring member is pivotable within the housing to vary an eccentricity of the ring member with respect to the rotor. The method may further include shifting a pivot position of the ring member from a first pivot position to a second pivot position displaced from the first pivot position.

In some example methods, the ring member is biased in a first pivot direction by engaging the ring member with a resilient element.

In some of the example methods, the radially extending vanes define one or more vane chambers configured to develop pressure during operation of the pump, the pressure in the one or more vane chambers imparting a force to the ring member offset from a pivot pin of the ring member by an offset distance. In these examples, the shifting of the pivot position may reduce the offset distance. In at least a subset of these examples, the offset is eliminated by the reduction in the offset distance.

In some example methods, shifting the pivot position includes shifting a pivot pin within the housing.

In at least some examples, a method includes providing first and second undulations in the housing corresponding to the first and second pivot positions of the ring member, respectively.

In other example methods, a cam may be rotatably fixed within the housing, wherein the pivot position is shifted by rotating the cam.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein:

FIG. 1 is a partial cutaway view of a variable displacement oil pump having a ring element with a movable pivot;

FIG. 2A is an example movable pivot for the variable displacement pump of FIG. 1;

FIG. 2B is another example of a movable pivot for the variable displacement pump of FIG. 1;

FIG. 2C is an illustration of a free-body diagram of the ring member of the pump of FIG. 1, illustrating internal torque and other forces applied to the ring member during operation of the pump;

FIG. 3A is a graph illustrating changes in internal torque of the variable displacement pump of FIG. 1 in response to shifts in a ring element pivot point using the movable pivots of FIG. 2A or 2B, according to an example illustration;

FIG. 3B is a graph illustrating changes in internal torque of the variable displacement pump of FIG. 1 in response to shifts in a ring element pivot point using the movable pivots of FIG. 2A or 2B, according to another example; and

FIG. 4 is an example process flow diagram, illustrating example methods for operating a variable displacement pump.

DETAILED DESCRIPTION

Turning now to FIG. 1, an example variable displacement pump 100 is illustrated. The pump 100 may include a housing 102 having an inlet and an outlet (not shown in FIG. 1A). The pump 100 may further include a rotor 104 that is fixed for rotation with a shaft 106. The shaft 106 may be rotatably mounted within the housing 102. The housing 102 may be an intermediate housing of an assembly forming a housing for the pump 100. A plurality of radially extending vanes 107 may be slidably disposed in the rotor 104. A pivotable ring member 108 defines a vane chamber 110 about the rotor 104. The ring member 108 may be pivotable within the housing 102, facilitating the varying of displacement output by way of altering an eccentricity of the ring member 108 with respect to the rotor 104.

The pump 100 also includes a biasing assembly 112. The biasing assembly 112 may apply a biasing force to the ring member 108 urging a lever 128 of the ring member 108 generally upward (in the orientation of FIG. 1), thereby tending to urge the ring member 108 to pivot in a first direction about a pivot pin 109 (as illustrated in FIG. 1, generally upwards). The biasing assembly 112 may include at least one resilient element 114 extending longitudinally. The resilient element, in one example, may be a coil spring formed of steel or aluminum, merely as examples.

The biasing assembly 112 may generally be used to control displacement of the pump 100 by way of positioning the ring member 108, along with other forces applied selectively to the ring member 108. More specifically, as seen in FIG. 1A, the ring element 108 may be eccentric with respect to the rotor 104. The ring member 108 is pivotable about a pivot pin 109. The pivot pin 109 may be provided by any mechanism that is convenient, as will be discussed further below. A plurality of vanes 107 are radially slidable in the rotor 107. A vane ring 105 may control the radial position of the vanes 107 within the rotor 104. Each of the vanes 107 contact the ring member 108 at a radially outermost end of the vane 107, and as such the ring member 108 generally controls the radial position of each of the vanes 107 with respect to the rotor 104. When the rotor 104 turns during operation, pressure builds within the vane chamber 110 of the pump 100 that is defined at least in part by the adjacent vanes 107, the rotor 104, and the ring member 108. Pressure builds within the chamber 110 as the rotor 104 turns, due to the volume restriction placed upon the chamber 110 by the ring member 108. The internal pressure that builds within the chamber 110 applies a torque to the ring member 108. More specifically, pressure building within the chamber 110 may apply a force to the ring member 108, due to an offset of the resultant force of the vane chamber(s) 110 from the pivot pin 109, as will be discussed further below. Thus, torque placed upon the ring member 108 by the pressure in the vane chamber(s) 110 may be in opposition to the force applied to the ring member 108 by the biasing assembly 112.

The position of the ring member 108 may be varied by application of an external force via a pressure within a control chamber 111. Thus, external pressure applied to the ring member by the control chamber 111 may act in opposition to the biasing force applied by the biasing assembly 112. The control chamber 111 may be defined at least in part by the housing 102 and the ring member 108. A slide seal 115 of the ring member 108 may facilitate buildup of pressure within the control chamber 111 by generally sealing between the ring member 108 and housing 102 as the ring member 108 pivots about the pivot pin 109, with the slide seal 115 sliding along the housing 102. The control chamber 111 may receive pressure, for example, from a return line (not shown) of an engine associated with the pump 100. When the ring member 108 is rotated to a position where it is concentric with respect to the rotor 104, the output of the pump 100 is zero displacement, as there is no difference in volume between the control chamber 110 and an exit port of the pump 100. By contrast, output displacement of the pump 100 is generally maximized when the ring member 108 is positioned at its greatest degree of eccentricity with respect to the rotor 104. As the external pressure applied to the ring member 108 may be controlled independently of the speed of the rotor 104, the external pressure may be modified during operation to position the ring member 108 (in opposition to the torque applied by the biasing assembly 112 and/or the internal pressure of the control chamber 110) to obtain a desired displacement output.

In the example illustrated in FIG. 1, an internal support surface 126 may provide a reaction surface for a first end of the resilient element 114 of the biasing assembly 112 (i.e., the end of the coil spring nearest surface 126) within the housing 102. A second end of the resilient element of the biasing assembly 112 opposite the first end may contact a lever 128 of the ring member 108. The lever 128 is generally fixed with the ring member 108 (and may be formed as a unitary part with the ring member 108) and extends radially away from the ring member 108. As such, the end of coil spring 114 contacting lever 128 may provide compression force from the coil spring 114. The biasing assembly 112 may thereby generally apply a biasing force to the ring member 108 in the first direction D.

As shown in FIG. 1, the pivot pin 109 of the ring member 108 may be fixed to the ring member 108, and may be movable in a first “x” direction as illustrated. The pivot pin 109 may also be movable in a second “y” direction that is perpendicular to the “x” direction. The position of the pivot pin 109 may be shifted in the x and/or y directions to reduce or eliminate an offset of the pivot pin 109 from the resultant force placed upon the ring member 108 by the vane chamber(s) 110 during operation, as will be discussed further below. Accordingly, the ring member 108 may pivot about different locations of the pivot pin 109. When the ring member 108 is pivoted about the pivot pin 109, the lever 128 may shift upward and downward generally, e.g., in a similar direction as the “x” direction.

As will be described further below, by shifting the pivot pin 109 the amount of an internal torque imparted to the ring member 108 by the forces created by the buildup of pressure within the vane chamber(s) 110 may be modified. For example, by shifting the pivot pin 109, an offset between the force imparted collectively upon the ring element 108 by the pressure within the vane chamber(s) 110 and the pivot pin 109 may be reduced or eliminated. Typically, shifting the pivot in the “x” direction as shown in FIG. 1 may have a larger impact in comparison to shifts of the pivot pin 109 in the “y” direction as shown in FIG. 1. Changes in the internal hydraulic torque may also be more significant than the change in spring torque during operation, and therefore can be used to regulate the pump pressure.

Shifting the pivot pin 109 may also adjust lever arms associated with forces applied to the ring member 108 by the resilient element 114. For example, shifting the pivot pin 109 in the “y” direction and away from the lever 128 (i.e., pivot pin 109 moves to the right in FIG. 1) may increase a distance of a lever arm formed by the lever 128 with respect to the pivot pin 109, thereby increasing the moment imparted by the resilient element 114 (which is generally in compression) upon the ring element 108 and increasing the degree to which the resilient element 114 may resist internal torque. Similarly, a moment imparted to the ring member 108 by external force applied by the control chamber 111 to the ring member 108 may generally be increased by shifting the pivot pin 109 further away from the lever 128 (i.e., pivot pin 109 moves to the right in FIG. 1). On the other hand, shifting the pivot pin 109 in the opposite direction (i.e., to the left and/or upward in FIG. 1) may generally have the opposite effect, as it would reduce the torque or moment imparted to the ring member 108 by a force placed upon the ring member 108 by pressure of the control chamber 111 or the resilient element 114. The lever 128 may have a resilient slide 140 or other laterally extendable member configured to allow limited lateral movement of the lever 128 when the pivot pin 109 is shifted laterally, i.e., in the y direction.

Shifting the pivot pin 109 in the “x” direction may, as noted above, alter an offset of the pivot pin 109 with respect to resultant forces placed upon the ring member 108 by pressure building within one or more of the vane chamber(s) 110. Thus, as the offset is reduced, any torque imparted to the ring member 108 by the force of the pressure within the vane chamber(s) 110 may be reduced. Similarly, where offset is increased, the torque imparted to the ring member 108 by the force of the pressure within the vane chamber(s) 110 may be increased.

Turning now to FIGS. 2A and 2B, example mechanisms for shifting the pivot pin 109 are described in further detail. Both of the example mechanisms include a pivot anchor 109 a or 109 b, which may be secured to the pivot pin 109 adjacent the ring member 108. Alternatively, the example pivot anchors 109 a, 109 b may be formed as a single piece with the ring member 108.

In the example of FIG. 2A, a first example of a pivot anchor 109 a includes a profiled anchor surface 200 a that is mated with a corresponding housing surface 202 a. The profiled anchor surface 200 a and housing surface 202 a may be shaped with different radii, such that the pivot pin 109 travels along a path P1 when the pivot anchor 109 a “rocks” with respect to the housing 102. As the pivot anchor 109 a rocks with respect to the housing 102, the pivot pin 109 moves in both “x” and “y” directions, thereby altering an offset of the pivot pin 109 with respect to forces placed upon the ring member 108 by pressure within the vane chamber(s) 110, and/or varying a lever arm of the forces discussed above that are placed upon the ring member 108 by the resilient element 114 and/or the control chamber 111. The pivot anchor 109 a and housing 102 may have corresponding gear teeth or other undulations to keep the pivot anchor 109 a and housing 102 engaged. For example, as illustrated in FIG. 2A, corresponding teeth 204 a and 204 b of the anchor surface 200 a and housing surface 202 a, respectively, are engaged when the lever 128 (not shown in FIG. 2A) of the ring member 108 is pivoted toward an upper end of the housing 102. By contrast, corresponding second teeth 206 a and 206 b of the anchor surface 200 a and housing surface 202 a, respectively, are engaged when the lever 128 (not shown in FIG. 2A) of the ring member 108 is pivoted in the opposite direction, i.e., toward a lower end of the housing 102. The pivot anchor 109 a and housing 102 may have, alternatively or in addition to the corresponding teeth/undulations, a high-friction material along the contacting surfaces of the pivot anchor 109 a and housing 102.

Turning now to FIG. 2B, another example pivot anchor 109 b is illustrated. The pivot anchor 109 b is fixed to the pivot pin 109, which extends from a rear face of the ring member 108 in similar fashion as described above with respect to pivot anchor 109 a. The pivot anchor 109 b, however, interfaces with a rotatable cam 210 along an anchor surface 208. The rotatable cam 210 is fixed translationally within the housing 102 and is rotatable with respect to the housing 102. Upon rotation of the cam 210, the pivot anchor 109 a translates along the path P₂, thereby shifting vertically (i.e., in the “x” direction) and laterally (i.e., in the “y” direction), thereby also shifting the ring member 108 (shown in phantom in FIG. 2B). As with the pivot anchor 109 a, when the pivot anchor 109 b moves via rotation of the cam 210 with respect to the housing 102, the pivot pin 109 moves in both “x” and “y” directions, thereby varying a lever arm of the forces placed upon the ring member 108, and/or altering an offset of the pivot pin 109 with respect to forces placed upon the ring member 108 by pressure within the vane chamber(s) 110 to cause a reduction in the internal torque. The cam 210 may be rotated by any mechanism that is convenient. Merely as examples, the cam 210 may be rotated by a hydraulic pressure or mechanism, or an electric solenoid.

Referring now to FIG. 2C, the forces applied to the ring member 108 during operation may include a spring force F_(S) imparted by the resilient element 114, external pressure applied by the control chamber 111, which creates an external force F₀ in opposition to the spring force F_(S), and an internal force F_(i) that is created by the buildup of pressure within the chambers 110 defined within the ring member 108. The internal force F_(i), due to the offset O with respect to the pivot pin 109, creates an internal torque τ_(I) acting upon the ring member 108, which acts in opposition to the spring force F_(S). By shifting the pivot pin 109 in the x and/or y directions, the pivot pin 109 of the ring element 108 may be shifted closer to a position where it is coincident with the resolved force F_(i) caused by the internal pressure of the vane chamber(s) 110, thereby reducing the internal torque τ_(I) placed upon the ring member 108 caused by the force F_(i). In other words, the closer the pivot position is to the line defined by the force F_(i), the less the internal torque τ_(I) upon the ring member 108 resulting from the force F_(i). Additionally, shifting of the pivot pin 109 may also alter lever arm(s) of (1) the force F_(O) imparted by the control chamber 111 and (2) the spring force F_(S) with respect to the pivot pin 109, thereby reducing/increasing the torque imparted upon the ring member 108 by those forces.

In the example pump 100 described above in FIGS. 1, 2A, and 2B, an eccentricity of the ring member 108 with respect to the rotor 104 may be varied independent of the rotating speed of the rotor 104. By comparison, in previous approaches a displacement of the pump 100 would become “self-regulated” by the pump at elevated speeds of the rotor 104. More specifically, at relatively high rotor 104 speeds, the internal pressure generated by the rotor 104 upon the ring member 108 (i.e., from the vane chamber(s) 110) becomes relatively large in comparison to the force of the resilient element 114 and/or the maximum pressure applied by the control chamber 111. Accordingly, the ring member 108 under such previous approaches cannot as easily be shifted to a desired position, particularly at elevated rotor 104 speeds. In fact, at extremely high pump speeds, the torque generated by the internal pressure on the ring element or slide in these previous approaches can overcome a maximum force applied by the control chamber, preventing control of ring element position.

By contrast, the example pump 100 described above allows the reduction of internal torque caused by the pressure within the ring member 108 by shifting the pivot pin 109. Internal torque of the ring member 108, i.e., that caused by the buildup of pressure within the chamber 110 during operation, may also be affected by rotor 104 speed, viscosity of oil being pumped through the pump 100, and displacement of the pump 100. With the reduced torque placed upon the ring member 108 when the pivot pin 109 is shifted, the eccentricity of the ring member 108 with respect to the rotor 104 may be more easily controlled by the external pressure applied via the control chamber 111. In one example, the internal torque generated by the chamber 110 upon the ring member 108 may be less than that imparted by the resilient element 114. With the increased control of displacement made possible in part by the reduced internal torque facilitated by the shiftable pivot pin 109, alternative control devices such as an oil control valve (OCV) may have greater effect, and in some cases may not be needed at all. In other words, in such examples lacking an oil control valve, the control chamber 111 may exclusively control position of the ring member 108, in opposition to the spring force F_(S), due to the reduced effect (i.e., torque) of the internal forces F_(i).

The increased control offered by exemplary pumps such as pump 100 described above is illustrated in the graphs illustrated in FIGS. 3A-3B. In each of FIGS. 3A and 3B, an internal torque caused by the force F_(i) is illustrated as a function of movement of the pivot pin 109 along the x and y directions, consistent with the orientations of x and y directions with respect to the pivot pin 109 illustrated in FIGS. 1, 2A, and 2B. The example pivot anchors 109 a, 109 b may facilitate a shifting of the pivot pin 109, allowing increased control of the displacement output of the pump 100. In a first example of a pump 100 illustrated in FIG. 3A, the internal torque acting upon ring member 108 that is caused by the internal force F_(i) may be varied from 1.4 Newton-meters (N-m) to 51 N-m by shifting the pivot pin 109 4.0 millimeters in either direction along the “x” axis from a baseline (“zero”) position, and 1.0 millimeters in either direction along the “y” axis from the baseline position. A second example of a pump 100 is illustrated in FIG. 3B, in which the internal torque acting upon ring member 108 that is caused by the internal force F_(i) may be varied from 2.3 Newton-meters (N-m) to 7.4 N-m by shifting the pivot pin 109 4.0 millimeters in either direction along the “x” axis from a baseline (“zero”) position, and 1.0 millimeters in either direction along the “y” axis from the baseline position.

Turning now to FIG. 4, an example process 400 of pumping a fluid, e.g., with a variable displacement pump, is described. Process 400 may being at block 410, where a housing may be provided. For example, as described above, a housing 102 may have an inlet and outlet for drawings in and expelling a fluid, e.g., oil. Process 400 may then proceed to block 420.

At block 420, a rotor may be rotatably mounted within the housing upon a shaft. In one example described above, rotor 104 may be mounted upon a shaft 106 for rotation within the housing 102.

Proceeding to block 430, a plurality of radially extending vanes may be positioned in the rotor. As described above, in some examples, vanes 107 may be provided, each of which are slidably disposed in the rotor 104 such that they may define a varying volume of each chamber 110 about the circumference of the rotor 104. Process 400 may then proceed to block 440.

At block 440, a control chamber may be defined about the rotor with a pivotable ring member. In an example as described above, a ring member 108 is provided, which is pivotable within the housing 102 to vary an eccentricity of the ring member 108 with respect to the rotor 104. Accordingly, varying amounts of displacement of the pump 100 may be provided, depending on the amount of eccentricity of the ring member 108 with respect to the rotor 104.

Proceeding to block 450, the ring member may be biased in a first pivot direction. For example, as described above the ring member 108 may be biased about the rotor in a first direction by way of a biasing assembly 112. In some example approaches, a biasing assembly 112 includes a resilient element 114, e.g., a coil spring. In some examples, the direction in which the biasing assembly 112 biases the ring element 108 is opposite to the direction in which pressure from the control chamber 111 urges the ring element 108. Process 400 may then proceed to block 460.

At block 460, a pivot position of the ring member may be shifted from a first pivot position to a second pivot position displaced from the first pivot position. In some example approaches consistent with those illustrated above, a ring member 108 may pivot about a pivot pin 109, which is shifted within the housing 102 to effect a corresponding shift in the axis of rotation of the ring member 108. The pivot position may be changed to reduce or increase a lever arm with respect to forces applied by the resilient element 114 and/or the control chamber 111, and/or reduce or increase an offset of the pivot pin 109 from a resultant force created by pressure within the vane chamber(s) 110. In another example, the pivot position may be shifted radially with respect to the rotor 104. A shift of the pivot pin 109 within the housing 102 may thus alter a moment or amount of torque imparted to the ring member 108 during operation. Accordingly, internal torque upon the ring member 108 may be reduced (or increased, if desired).

In some examples, the housing 102 may define a varying surface profile, e.g., with a plurality of teeth 204 b, 206 b, e.g., as illustrated above in FIG. 2A, or other undulations. In these examples, this “rockered” profile shifts the pivot pin 109 when the ring member 108 is rotated.

In other examples, a rotatable cam 210 may facilitate shifting a pivot position of the ring member 108 independently of the ring member 108 position about the rotor 104, such as in the example of FIG. 2B. In other words, in contrast to the rockered profile illustrated in FIG. 2A, which depends upon pivoting of the ring member 108 to shift the pivot position, the cam 210 may be rotated independently of the ring member 108 position. Merely as examples, the cam 210 may be rotated by hydraulic pressure or electrically actuated, e.g., via a solenoid.

It is to be understood that the foregoing is a description of one or more embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims.

As used in this specification and claims, the terms “e.g.,” “for example,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation. 

What is claimed is:
 1. A variable displacement pump, comprising: a housing having an inlet and an outlet; a rotor fixed for rotation with a shaft, the shaft rotatably mounted within the housing; a plurality of radially extending vanes slidably disposed in the rotor; and a pivotable ring member defining at least in part a control chamber about the rotor, wherein the ring member is pivotable within the housing to vary an eccentricity of the ring member with respect to the rotor, wherein the ring member is configured to shift a pivot position of the ring member from a first pivot position to a second pivot position displaced from the first pivot position.
 2. The pump of claim 1, further comprising a resilient element engaged with the ring member, thereby biasing the ring member in a first pivot direction opposed to a second pivot direction, the control chamber positioned such that an external pressure introduced into the control chamber applies a force to the ring member tending to pivot the ring member in the second pivot direction.
 3. The pump of claim 1, wherein the radially extending vanes define one or more vane chambers configured to develop pressure during operation of the pump, the pressure in the one or more vane chambers imparting a force to the ring member offset from a pivot pin of the ring member by an offset distance, wherein the pivot position is shiftable to reduce the offset distance.
 4. The pump of claim 1, further comprising a pivot pin configured to define the first and second pivot positions.
 5. The pump of claim 4, further comprising a pivot anchor fixed to the ring member.
 6. The pump of claim 1, wherein the housing defines at least first and second undulations corresponding to the first and second pivot positions of the ring member, respectively.
 7. The pump of claim 1, further comprising a cam rotatably fixed within the housing.
 8. The pump of claim 7, wherein the cam defines the first pivot position of the ring member when placed in a first rotational position, and the cam defines the second pivot position of the ring member when rotated from the first rotational position to a second rotational position.
 9. A variable displacement pump, comprising: a housing having an inlet and an outlet; a rotor fixed for rotation with a shaft, the shaft rotatably mounted within the housing; a plurality of radially extending vanes slidably disposed in the rotor; a pivotable ring member defining at least in part a control chamber about the rotor, wherein the ring member is pivotable within the housing to vary an eccentricity of the ring member with respect to the rotor, wherein the ring member is configured to shift a pivot position of the ring member from a first pivot position to a second pivot position displaced from the first pivot position; and a resilient element engaged with the ring member, thereby biasing the ring member in a first pivot direction about the first and second pivot positions.
 10. The pump of claim 9, wherein the radially extending vanes define one or more vane chambers configured to develop pressure during operation of the pump, the pressure in the one or more vane chambers imparting a force to the ring member offset from a pivot pin of the ring member by an offset distance, wherein the pivot position is shiftable to reduce the offset distance.
 11. The pump of claim 9, further comprising a pivot pin configured to define the first and second pivot positions.
 12. The pump of claim 9, wherein the housing defines first and second undulations corresponding to the first and second pivot positions of the ring member, respectively.
 13. The pump of claim 9, further comprising a cam rotatably fixed within the housing.
 14. A method of pumping a fluid, comprising: providing a housing having an inlet and an outlet; rotatably mounting a rotor within the housing upon a shaft; positioning a plurality of radially extending vanes in the rotor, the vanes each slidably disposed in the rotor; defining a control chamber about the rotor with a pivotable ring member, wherein the ring member is pivotable within the housing to vary an eccentricity of the ring member with respect to the rotor; and shifting a pivot position of the ring member from a first pivot position to a second pivot position displaced from the first pivot position.
 15. The method of claim 14, further comprising biasing the ring member in a first pivot direction by engaging the ring member with a resilient element.
 16. The method of claim 14, wherein the radially extending vanes define one or more vane chambers configured to develop pressure during operation of the pump, the pressure in the one or more vane chambers imparting a force to the ring member offset from a pivot pin of the ring member by an offset distance; and wherein shifting the pivot position reduces the offset distance.
 17. The method of claim 16, wherein the offset is eliminated by the reduction in the offset distance.
 18. The method of claim 14, wherein shifting the pivot position includes shifting a pivot pin within the housing.
 19. The method of claim 14, further comprising providing first and second undulations in the housing corresponding to the first and second pivot positions of the ring member, respectively.
 20. The method of claim 14, further comprising providing a cam rotatably fixed within the housing, wherein the pivot position is shifted by rotating the cam. 